This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Synchrotron beamtime is requested for X-ray crystallographic studies of human DNA topoisomerase I (topo I). Topo I is an essential enzyme in DNA replication, and the target of anti-cancer drugs. The synchrotron beamtime will be used to determine structures of complexes of topo I with small molecule compounds for design and development of new topo I drugs.